Free piston cryogenic refrigerator with phase angle control

ABSTRACT

Cryogenic refrigerator free piston displacer reciprocates as a function of cyclic operating pressure, seal drag and bounce cylinder pressure and volume. Bounce cylinder volume is established for a particular pressure pulse frequency to optimize phase angle.

[22] Filed:

United States Patent Leo [451 Apr. 15, 1975 1 FREE PISTON CRYOGENICREFRIGERATOR WITH PHASE ANGLE CONTROL [75] Inventor: Bruno S. Leo, SantaMonica, Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

Mar. 18, 1974 [21] Appl. No.: 452,033

[52] US. Cl 62/6; 62/86 [51] Int. Cl. F25b 9/00 [58] Field of Search62/6, 86

[56] References Cited UNITED STATES PATENTS 3,119,237 1/1964 Gifford62/6 3,188,821 6/1965 Chellis 62/6 3,312,072 4/1967 Gifford 62/63,321,926 5/1967 Chellis 62/6 3,620,029 11/1971 Longsworth 62/6 PrimaryExaminerWilliam J. Wye Attorney, Agent, or FirmAllen A. Dicke, Jr.; W.H. MacAllister [57] ABSTRACT Cryogenic refrigerator free pistondisplacer reciprocates as a function of cyclic operating pressure, sealdrag and bounce cylinder pressure and volume. Bounce cylinder volume isestablished for a particular pressure pulse frequency to optimize phaseangle.

4 Claims, 3 Drawing Figures maximums 77, 239

Fig.1. 46 44 Fig. 5. H 2

V28 Vl6 FREE PISTON CRYOGENIC REFRIGERATOR WITH PHASE ANGLE CONTROLBACKGROUND OF THE INVENTION This invention is directed to a free pistoncryogenic refrigerator, and particularly one where the spring constantfor returning the cold displacer is conditioned upon structural andoperating criteria to cause the cold displacer to operate in an optimumcyclic condition.

Free piston cryogenic refrigerators have a pulse generator whichgenerates cyclic pressure pulses of refrigerant gas. A reciprocatingcold cylinder displacer acts as an expander and responds to the pulsesby reciprocation within the cold cylinder. Response to the pulses isaccomplished by having a larger area on the cold end of the colddisplacer than on the ambient end, and have a balance or bounce springon the warm end.

Higa patent US. Pat. No. 3,367,121 is an example of such structure whichemploys a Stirling pulse generator. Furthermore, the Robert Berry andAxel Dehne application Ser. No. 447,417, filed Mar. 1, 1974 entitledVUILLEUMIER REFRIGERATOR WITI-I SEPA- RATE PNEUMATICALLY OPERATED COLDDIS- PLACER is an example of such a structure when a VM pulse cyclepressure source is used.

SUMMARY OF THE INVENTION In order to aid in the understanding of thisinvention it can be stated in essentially summary form that it isdirected to a free piston refrigerator which has control of its phaseangle so that the phase angle between piston stroke and input pressurecycles is substantially 90 and is preferrably at optimum refrigerationefficiency. This is accomplished by controlling the spring rate of thecold displacer bounce spring so that the cold displacer operates at theproper phase angle when the correct pressure pulse input frequency isapplied to the warm end of the cold cylinder.

It is thus an object of this invention to define and provide the properreturn force on a free piston cold displacer so that the piston operatesat the optimum phase angle with respect to the power pulse.

Other objects and advantages of this invention will become apparent fromthe study of the following portion of the specification, the claims andthe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partly sectional andpartly schematic diagram of the free piston refrigerator with phaseangle control of this invention.

FIG. 2 is a PV diagram of the pulse generator of this invention.

FIG. 3 is a PV diagram of the cold end of the cold cylinder.

DESCRIPTION This invention is directed to a free piston cryogenicrefrigerator apparatus with phase angle control of the free piston withrespect to the pressure pulses from the refrigerant gas pulse generator.

It is applicable to those reciprocating machines where the motions ofthe compressor, whether it be a mechanical or thermal compressor, andthe expander are not mechanically linked. Modified Stirlingrefrigerators are shown in the invention by W. H. Higa, US. Pat. No.3,421,331 particularly FIG. 5, and in G. Prast,

US. Pat. No. 3,487,635. An example of the modified VM refrigerator isshown in Robert Berry and Axel Dehne patent application Ser. No. 447,417filed Mar. 1, 1974 entitled Vuilleumier Refrigerator with SeparatePneumatically Operated Cold Displacer, and Bruno S. Leo application Ser.No. 449,182, filed Mar. 7, 1974 for VM Cryogenic Refrigerator I-IotCylinder Burner Head.

In general, modified Stirling and modified VM refrigerators havecompressor and expander sections which are separated, except for a gaspassage between the two sections. Motion of the cold displacer piston iscontrolled by input pulse frequency, pressure, free piston mass, pistonarea, size and length of the connecting line between the warm end of thefree piston and the compressor cylinder and the spring constant of thereturn spring. In a practical design, quite a number of these factorsare fixed by other. criteria. For example, the input frequency,pressure, free piston mass and the connecting line size are determinedby other factors. It is the spring constant of the return spring whichis most convenient of adjustment.

FIG. 1 illustrates a modified Stirling refrigerator l0. Refrigerator 10comprises a compressor 12 which has a piston 14 reciprocating incompressor cylinder 16. Reciprocation is caused by crank 18 which isdriven by an appropriate motor. This produces pressure pulses ofrefrigerant gas in line 20. For purposes of phase angle considerationsthere is no phase delay between operation of piston 14 and the pressurepulses at inlet 21 to warm volume 36.

The expander is generally indicated at 22. The expander comprises anexpander cylinder 24 in which is mounted a reciprocating expander piston26. Volume 28 is the expansion volume in which the refrigerant gas isexpanded to produce refrigeration. Cold cylinder head 30 is the point ofrefrigeration and is the location upon which refrigerated devices suchas IR device 31 are mounted. Window 32 in insulator housing 34 permitsthe refrigerated device, such as infrared sensitive device 31, to havea-field of view external to the insulator housing. The insulator housingcan be in the nature of a dewar or the like. Volume 36 is an ambientvolume to which the line 20 is connected. Volume 36 is connected throughthe interior of cold piston 26 through regenerator 38 to cold volume 28.Sliders or guides on the exterior of the expander piston permit thepiston to slide within the expander cylinder. There is no pressure dropalong the length of the expander piston, except for the pressure dropthrough the regenerator, so that the sliders do not have a substantialseal duty.

Balance piston 40 is mounted on the expander piston and is sealed inbalance cylinder 42. The upper end of the balance cylinder 42 is springvolume 44.

Calculations show:

tan 0 2d 0: co V1 (com where:

O phase angle between the free piston and the input pressure pulse tothe warm volume d f/(2 V km); f free piston friction factor 0)compressor piston frequency to natural frequency of the free piston(k/m)" k spring constant of piston return spring-chamber 44 k springconstant of gas line 20 and of free piston cylinder 24 1 m mass of freepiston.

In designing free piston refrigerators having a free piston expandersuch as indicated at 22, several of the parameters or criteria come outas parts that are fixed by other design criteria. For example, inputfrequency, pressure, free piston mass and the size and length of theconnecting line between the free piston and the pulse generator arequantities which are fairly fixed by the physical conditions of theinstallation, the net refrigeration required and the temperature of thecold point are similar parameters. The optimum phase angle is about 90,but this angle is seldom attained in the final design unless particularcare is taken in the critical design of the spring 44 to attain thedesired spring constant. The spring constant in the bouncing chamber 44is determined in accordance with the mathematical expression above, andthe chamber is then designed to achieve that spring constant. In orderto permit convenient and empirical adjustment of the spring constant,both to reduce the criticality of the original manufacture of thisspring bounce chamber 44 and to permit critical adjustment ofrefrigerator operation to maximum operating efficiency even if it is notexactly at a 90 phase angle relationship, tube 46 is attached to thespring bounce chamber 44 to act as a part thereof. The volume of thetube is adjusted in size until the refrigerator is tuned to maximumperformance. This adjustment may he means of a variable volume tube by atelescoping structure, or it may be removed and cut at each test, or itmay be simply pinched closed for a short length along the closed end toreduce the volume until the maximum performance is achieved.

Referring to the operation of the cycle, FIG. 2 is a PV diagram of thecompressor. With a compressor piston at top dead center point 47, thecold displacer is assumed to be at bottom dead center, as at point 48which is the maximum cold volume. Since the pressure in the coldcylinder is now higher than the mean pressure in the pneumatic springvolume 44, a force acts on the cold displacer tending to hold it in thebottom dead 4 the system pressure decreases steadily and reaches the 4mean pressure at point 50 approximately at the end of this quarterrevolution. The corresponding point in FIG. 3 is at point 52 where thecold displacer remains at the bottom dead center position. The pressurewhich maintains the cold displacer in this position has dropped to zeroat point 52. During the next quarter revolution, the compressor pistoncontinues to move toward bottom dead center, and the system pressurecontinues to drop. Since the pressure in the expansion volume hasdecreased below the mean pressure in the pneumatic spring volume 44, anactivation force has developed. When this force exceeds the frictionaldrag of the seals and the pressure drop through the regenerator, thecold displacer will move towards its top dead position 54, as seen inFIG. 3. Below the point 52, the piston starts to move, and, as thepressure in line decreases to its condition of bottom dead center point56, the cold displacer 26 moves to its top dead center point 54, asillustrated in FIG. 3. At the end of the half cycle, the compressorpiston is at bottom dead center and the cold displacer is at top deadcenter. In this position, pressure in the system is near minimum. Thecold displacer is again ina stable position, as being held in its topdead center position.

During the third quarter rotation of the crank, the compressor pistonagain moves toward top dead center from point 56 past point 58 to thetop dead center point 47. During this motion, the system pressureincreases steadily and reaches approximately the mean pressure at thequarter cycle point 58. The corresponding pressure is shown at point 60in FIG. 3. During the final quarter cycle, the compressor pistoncontinues to move toward top dead center and the system pressurecontinues to rise. When the pressure rises above the mean cyclic value;that is, above points 58 and 60, a resultant force is developed on thecold displacer 26. When this force exceeds frictional drag on the seals,the cold displacer will move toward bottom dead center, as illustratedat the curve above point 60 in FIG. 3. At the end of this cycle, thecold displacer is at bottom dead center and the compressor piston is attop dead center; thus, the cycle is complete. The area enclosed by theindicated PV diagram of the expander, FIG. 3, is the work performed bythe gas on the cold displacer and is equal to the gross refrigerationdeveloped at the expander.

This invention having been described in its preferred embodiment, it isclearly susceptible to numerous modifications and embodiments within theability of those' skilled in the art and without the exercise of theinventor faculty. Accordingly, the scope of this invention is defined bythe scope of the following claims.

What is claimed is:

l. A cryogenic refrigerator comprising:

a pulse tube, means for cyclically generating a refrigerant gas pressurepulse in said pulse tube;

a free piston expander comprising a cylinder having a cold end and awarm end, a free piston mounted in said expander cylinder to separatesaid cylinder into a cold volume and a warm volume, the area of saidpiston facing said warm volume being of smaller area than the cold endof said free piston facing said cold volume, said pulse tube beingconnected to said warm volume, the improvement comprising:

a spring urging said piston toward said cold end to reduce said coldvolume, said spring having a spring constant such that the reciprocationof said free piston in said expander cylinder has an optimumsubstantially phase angle relationship with respect to refrigerant gaspressure pulses delivered to said warm volume by said pressure pulsetube.

2. The cryogenic refrigerator of claim 1 wherein said spring urging saidfree piston to reduce said cold volume is a pneumatic spring.

3. The cryogenic refrigerator of claim 2 wherein said free piston has abounce piston formed on the warm end thereof, said bounce pistonengaging in a bounce cylinder, said bounce piston having a smaller areathan the area of said free piston at its cold end, said bounce cylinderbeing connected to a pneumatic bounce chamber, the spring constant ofsaid bounce chamber causing said free piston to operate at an optimumsubstantially 90 phase angle with respect to incoming pressure pulsecycles.

4. The cryogenic refrigerator of claim 3 wherein said bounce chamber hasa spring constant to produce the physical relationship:

phase angle between the free piston and the input k spring constant ofpiston return spring-chamber pressure pulse to the warm volume 44 d f/(2Vkm); f free piston friction factor k spring constant of gas line 20 andof free piston w compressor piston frequency cylinder 24 (o naturalfrequency of the free piston (k/m)" 5 m mass of free piston. k k k

1. A cryogenic refrigerator comprising: a pulse tube, means forcyclically generating a refrigerant gas pressure pulse in said pulsetube; a free piston expander comprising a cylinder having a cold end anda warm end, a free piston mounted in said expander cylinder to separatesaid cylinder into a cold volume and a warm volume, the area of saidpiston facing said warm volume being of smaller area than the cold endof said free piston facing said cold volume, said pulse tube beingconnected to said warm volume, the improvement comprising: a springurging said piston toward said cold end to reduce said cold volume, saidspring having a spring constant such that the reciprocation of said freepiston in said expander cylinder has an optimum substantially 90* phaseangle relationship with respect to refrigerant gas pressure pulsesdelivered to said warm volume by said pressure pulse tube.
 2. Thecryogenic refrigerator of claim 1 wherein said spring urging said freepiston to reduce said cold volume is a pneumatic spring.
 3. Thecryogenic refrigerator of claim 2 wherein said free piston has a bouncepiston formed on the warm end thereof, said bounce piston engaging in abounce cylinder, said bounce piston having a smaller area than the areaof said free piston at its cold end, said bounce cylinder beingconnected to a pneumatic bounce chamber, the spring constant of saidbounce chamber causing said free piston to operate at an optimumsubstantially 90* phase angle with respect to incoming pressure pulsecycles.
 4. The cryogenic refrigerator of claim 3 wherein said bouncechamber has a spring constant to produce the physical relationship: tantheta 2d omega omega o 1/1 - ( omega omega o 1)2 where: theta phaseangle between the free piston and the input pressure pulse to the warmvolume d f/(2 Square Root km); f free piston friction factor omegacompressor piston frequency omega o natural frequency of the free piston(k/m)1/2 k k1 + k2 k1 spring constant of piston return spring-chamber 44k2 spring constant of gas line 20 and of free piston cylinder 24 m massof free piston.